Protein omega from Escherichia coli

A highly processive actinobacterial topoisomerase I - thoughts on Streptomyces' demand for an enzyme with a unique C-terminal domain

Topoisomerase I (TopA) is an essential enzyme that is required to remove excess negative supercoils from chromosomal DNA. Actinobacteria encode unusual TopA homologues with a unique C-terminal domain that contains lysine repeats and confers high enzyme processivity. Interestingly, the longest stretch of lysine repeats was identified in TopA from Streptomyces, environmental bacteria that undergo complex differentiation and produce a plethora of secondary metabolites. In this review, we aim to discuss potential advantages of the lysine repeats in Streptomyces TopA. We speculate that the chromosome organization, transcriptional regulation and lifestyle of these species demand a highly processive but also fine-tuneable relaxase. We hypothesize that the unique TopA provides flexible control of chromosomal topology and globally regulates gene expression.

ATP-independent DNA topoisomerase from Fervidobacterium islandicum

Thermotogales are thermophilic eubacteria belonging to a very slowly evolving branch in the eubacterial tree. In this report, we describe the purification and characterization of an ATP-independent DNA topoisomerase from the Thermotogale, Fervidobacterium islandicum. The enzyme, a monomer of about 75 kDa, is a type I DNA topoisomerase sharing many properties with the other bacterial topoisomerases I: it absolutely requires Mg2+ for activity, relaxes negatively but not positively supercoiled DNA and is inhibited by single-stranded M13 DNA and spermidine. A feature of the F. islandicum ATP-independent DNA topoisomerase I is its thermophily. The optimal temperature for the enzymatic activity is 75 degrees C. Studies about thermostability show that the enzyme is more stable when incubated undiluted in the storage buffer. In these conditions, 60% activity was retained after a 30 min preincubation at 75 degrees C.

Differential modulation by spermidine of reactions catalyzed by type 1 prokaryotic and eukaryotic topoisomerases

In the absence of DNA aggregation, spermidine inhibited the relaxation of negatively supercoiled DNA by Escherichia coli topoisomerase I at concentrations of the polyamine normally found intracellularly. Spermidine also curtailed the cleavage of negatively supercoiled ColE1 DNA by the enzyme in the absence of Mg2+. On the contrary, knotting of M13 single-stranded DNA circles catalyzed by topoisomerase I was stimulated by the polyamine. Relaxation of supercoiled DNA by eukaryotic type 1 topoisomerases, such as calf thymus topoisomerase I and wheat germ topoisomerase, was significantly stimulated by spermidine in the same range of concentrations that inhibited the prokaryotic enzyme. In reactions catalyzed by S1 nuclease, the polyamine enhanced the digestion of single-stranded DNA and inhibited the nicking of negatively supercoiled DNA. These results suggest that spermidine modifies the supercoiled duplex substrate in these reactions by modulating the degree of single strandedness.